March 15, 2013 - UNITED STATES - More than 10,000 people could die when -- not if -- a monster earthquake and tsunami occur just off the Pacific Northwest coast, a new study predicts.
Coastal towns would be inundated. Schools, buildings and bridges would collapse, and economic damage could hit $32 billion.
The chilling report by the Oregon Seismic Safety Policy Advisory Commission was presented Thursday to Oregon legislators.
In 2011, the Legislature authorized the study of what would happen if a quake and tsunami such as the one that devastated Japan hit the Pacific Northwest.
The Cascadia Subduction Zone, just off the regional coastline, produced a megaquake in the year 1700. Seismic experts say another monster quake and tsunami are overdue.
"This earthquake will hit us again," Kent Yu, an engineer and chair of the commission, told legislators. "It's just a matter of how soon."
When it hits, the report says, there will be devastation and death from Northern California to British Columbia.
Many Oregon communities will be left without water, power, heat and telephone service. Gasoline supplies will be disrupted.
The 2011 Japan quake and tsunami were a wake-up call for the Pacific Northwest. Governments have been taking a closer look at whether the region is prepared for something similar and discovering it is not.
Oregon legislators requested the study so they could better inform themselves about what needs to be done to prepare and recover from such a giant natural disaster.
The report says that geologically, Oregon and Japan are mirror images. Despite the devastation in Japan, that country was more prepared than Oregon because it had spent billions on technology to reduce the damage, the report says.
Jay Wilson, who works for the Clackamas County emergency management department and is vice chair of the commission, visited Japan and said he was profoundly affected as he walked through villages ravaged by the tsunami.
"It was just as if these communities were ghost towns and for the most part there was nothing left," he said.
Wilson told legislators that there was a similar event 313 years ago in the Pacific Northwest, and "we're well within the window for it to happen again" - Oregon Live.
The subduction of the Juan de Fuca plate beneath North America changes markedly along the length of the subduction zone, notably in the angle of subduction, distribution of earthquakes, volcanism, geologic and seismic structure of the upper plate, and regional horizontal stress. To investigate these characteristics, we conducted detailed density modeling experiments of the crust and mantle along two transects across the Cascadia subduction zone. One crosses Vancouver Island and the Canadian margin, and the other crosses the margin of central Oregon. Both density models were constructed independently to a depth of approximately 50 km. We gathered all possible geologic, geophysical, and borehole data to constrain the density calculations. The final densities for the Oregon and Vancouver lithosphere models were obtained from gravity inversions.
Our results confirm that the downgoing slab of the Cascadia subduction zone dips significantly steeper beneath Oregon than beneath Vancouver Island, lending support to the idea that the Juan de Fuca plate is segmented from north to south. In addition, our gravity models indicate that the mantle wedge beneath western Oregon (i.e., below the western Cascades) is lighter than the mantle beneath the Canadian continental crust. This low density agrees with the low mantle velocities observed in the mantle and the present day extensional regime of the Pacific Northwest.
A gravity low at the deformation front of the Oregon margin, absent along the Vancouver margin, can be explained by the different bathymetry of the two regions and by the depth to the top of the subducting Juan de Fuca plate. If the accretionary prisms along these profiles were modeled with equal densities, a density inhomogeneity in the lower part of the models would be necessary. Thus that the density of the accretionary prism for the Vancouver profile must be approximately 0.1-0.2 g/cm3 greater than that for Oregon. A density difference within the accretionary prisms also agrees with other data. We note that the volume of accreted sediments is approximately twice as large along the Vancouver profile than along the Oregon profile, and the prism reaches a greater depth (approximately 20 km as compared with 12 km for the Oregon profile). This implies that the sediments within the accretionary prism at Vancouver Island are at a higher metamorphic grade, and therefore have higher densities.
We find that a substantial part of the coastal gravity maxima for both lines is caused by increasing density with depth in the subducting plate. In the proposed model, the maximum possible density of the slab was used to satisfy constraints for the average density of the near coastal crust for both profiles. If a density increase with depth is not introduced into the model, very high densities would be required for the near surface coastal and continental crustal blocks. - USGS.
Coastal towns would be inundated. Schools, buildings and bridges would collapse, and economic damage could hit $32 billion.
 |
| Seawater
hits the city of Seaside on the Oregon coast in a simulation at the O.
H, Hinsdale Wave Research Laboratory at Oregon State University. The
event will be brought about by the Cascadia Subduction Zone thrust fault
approximately 50 miles offshore. The USGS says there is a 14% chance of
this event occurring in the next 15 years. Such events have happened
along the Oregon coast every 500 years. |
In 2011, the Legislature authorized the study of what would happen if a quake and tsunami such as the one that devastated Japan hit the Pacific Northwest.
The Cascadia Subduction Zone, just off the regional coastline, produced a megaquake in the year 1700. Seismic experts say another monster quake and tsunami are overdue.
"This earthquake will hit us again," Kent Yu, an engineer and chair of the commission, told legislators. "It's just a matter of how soon."
When it hits, the report says, there will be devastation and death from Northern California to British Columbia.
Many Oregon communities will be left without water, power, heat and telephone service. Gasoline supplies will be disrupted.
The 2011 Japan quake and tsunami were a wake-up call for the Pacific Northwest. Governments have been taking a closer look at whether the region is prepared for something similar and discovering it is not.
Oregon legislators requested the study so they could better inform themselves about what needs to be done to prepare and recover from such a giant natural disaster.
The report says that geologically, Oregon and Japan are mirror images. Despite the devastation in Japan, that country was more prepared than Oregon because it had spent billions on technology to reduce the damage, the report says.
Jay Wilson, who works for the Clackamas County emergency management department and is vice chair of the commission, visited Japan and said he was profoundly affected as he walked through villages ravaged by the tsunami.
"It was just as if these communities were ghost towns and for the most part there was nothing left," he said.
Wilson told legislators that there was a similar event 313 years ago in the Pacific Northwest, and "we're well within the window for it to happen again" - Oregon Live.
Cascadia Subduction Zone: Two Contrasting Models of Lithospheric Structure.
The subduction of the Juan de Fuca plate beneath North America changes markedly along the length of the subduction zone, notably in the angle of subduction, distribution of earthquakes, volcanism, geologic and seismic structure of the upper plate, and regional horizontal stress. To investigate these characteristics, we conducted detailed density modeling experiments of the crust and mantle along two transects across the Cascadia subduction zone. One crosses Vancouver Island and the Canadian margin, and the other crosses the margin of central Oregon. Both density models were constructed independently to a depth of approximately 50 km. We gathered all possible geologic, geophysical, and borehole data to constrain the density calculations. The final densities for the Oregon and Vancouver lithosphere models were obtained from gravity inversions.
Our results confirm that the downgoing slab of the Cascadia subduction zone dips significantly steeper beneath Oregon than beneath Vancouver Island, lending support to the idea that the Juan de Fuca plate is segmented from north to south. In addition, our gravity models indicate that the mantle wedge beneath western Oregon (i.e., below the western Cascades) is lighter than the mantle beneath the Canadian continental crust. This low density agrees with the low mantle velocities observed in the mantle and the present day extensional regime of the Pacific Northwest.
 |
| View the Vancouver or the Oregon profile at a larger scale annotated with density values. |
A gravity low at the deformation front of the Oregon margin, absent along the Vancouver margin, can be explained by the different bathymetry of the two regions and by the depth to the top of the subducting Juan de Fuca plate. If the accretionary prisms along these profiles were modeled with equal densities, a density inhomogeneity in the lower part of the models would be necessary. Thus that the density of the accretionary prism for the Vancouver profile must be approximately 0.1-0.2 g/cm3 greater than that for Oregon. A density difference within the accretionary prisms also agrees with other data. We note that the volume of accreted sediments is approximately twice as large along the Vancouver profile than along the Oregon profile, and the prism reaches a greater depth (approximately 20 km as compared with 12 km for the Oregon profile). This implies that the sediments within the accretionary prism at Vancouver Island are at a higher metamorphic grade, and therefore have higher densities.
We find that a substantial part of the coastal gravity maxima for both lines is caused by increasing density with depth in the subducting plate. In the proposed model, the maximum possible density of the slab was used to satisfy constraints for the average density of the near coastal crust for both profiles. If a density increase with depth is not introduced into the model, very high densities would be required for the near surface coastal and continental crustal blocks. - USGS.
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